Semiconductor device capable of threshold voltage adjustment by applying an external voltage

ABSTRACT

A semiconductor device has a silicon substrate, in which an active region is formed between two device isolation films and a gate is formed on the surface of the active region. The silicon substrate has a laterally etched portion in the active region below the surface of the active region on the side near the device isolation film. An insulating film is formed on the laterally etched portion of the silicon substrate. A conductive electrode is formed on the insulating film, through which an external voltage is applied to adjust a threshold voltage. The device isolation film is formed on the conductive electrode. None or some pockets of vacant cavity is present between the device isolation film and the conductive electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a semiconductor devicecapable of adjusting the threshold voltage for optimal deviceoperations, and more particularly to a semiconductor device capable ofthreshold voltage adjustment by applying an external voltage and amethod for manufacturing the same by which a partial SOI structure isrealized.

2. Description of the Prior Art

As generally known, a MOSFET in a semiconductor device operatessensitively in response to a threshold voltage (Vt). In order to obtainan optimal threshold voltage value of the MOSFET, issues relating tooptimization of impurity implantation and/or thermal processes, and thelike, are becoming important issues in the field of semiconductormanufacturing processes.

As semiconductor devices are miniaturized due to high-integration, moreexcessive impurity implantation is conventionally required in order toadjust the threshold voltage to an appropriate value. This excessiveimpurity implantation for purposes of adjusting the threshold voltageinevitably leads to deterioration of the device refresh characteristicsand lowering of the device reliability due to the undesired dopantdiffusion. Consequently, the conventional technique of excessivelyimplanting impurity to adjust the threshold voltage does not lead tosatisfactory results, especially for a highly integrated device.

Another conventional method applies a back bias to the body of a devicefor purposes of adjusting the threshold voltage. However, thisconventional technique too has limitations, because the back bias willhave less and less influence on the body as the semiconductor devicesize becomes small. In particular, since the channel and the bodyregions are surrounded by gates in a multiple gate structure (such as adouble gate, a triple gate, or a surrounding gate structure) the backbias will have no influence on the body. The result is that it isimpossible to adjust the threshold voltage by applying the back bias.

In conclusion, it is difficult to obtain an optimal threshold voltage ina highly integrated semiconductor device by applying the conventionaltechniques of the impurity implantation or thermal or other conventionalprocesses. This presents challenges to securing optimal devicecharacteristics coincident with the high integration of thesemiconductor devices.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve theabove-mentioned problem occurring in the prior art, and an object of thepresent invention is to provide a semiconductor device and itsmanufacturing method, in which an appropriate threshold voltage can beobtained.

A further object of the present invention is to provide a semiconductordevice and its manufacturing method, in which desired devicecharacteristics can be secured by adjusting an appropriate thresholdvoltage.

In order to accomplish these objects, there is provided a semiconductordevice in accordance with one aspect of the present invention, thesemiconductor device comprising: a silicon substrate having a deviceisolation film for defining an active region; a gate being formed on theactive region of the silicon substrate; and junction regions beingformed on a substrate surface at both sides of the gate, wherein thesilicon substrate includes a vacant space adjoining the device isolationfilm within the active region, and a surface of the vacant space isformed with a conductive electrode to which a voltage for adjusting thepotential of a substrate body region is externally applied while aninsulating film is interposed between the body region substrate and theconductive electrode.

In accordance with another aspect of the present invention, there isprovided a method for manufacturing a semiconductor device, the methodcomprising the steps of: forming a pad oxide film and a pad nitride filmon a silicon substrate having a device isolation region and an activeregion; etching the pad oxide film, the pad nitride film and the siliconsubstrate to form a trench in the device isolation region; forming aninsulating film spacer on a trench sidewall including the etched padoxide and nitride films; performing lateral etch from a siliconsubstrate portion of an exposed bottom surface of the trench toward theactive region to form a vacant space within the active region of thesilicon substrate; removing the insulating film spacer; forming aconductive electrode by interposing insulating film on a surface of thevacant space; filling up the trench with an oxide film to form a deviceisolation film; forming a gate on the silicon substrate; and formingjunction regions on a substrate surface at both sides of the gate.

Preferably, the method for manufacturing a semiconductor deviceaccording to the present invention further comprises the step ofperforming heat treatment at a temperature of 800 to 1000° C. under ahydrogen atmosphere after the step of forming the insulating film spacerand before the step of forming the vacant space within the active regionof the silicon substrate.

Also, it is preferred that the manufacturing method according to thepresent invention further comprises the step of performing field stopimplantation onto the substrate surface of the exposed trench bottomsurface after the step of forming the vacant space within the activeregion of the silicon substrate and before the step of removing theinsulating film spacer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 shows a sectional view of a semiconductor device in accordancewith a preferred embodiment of the present invention; and

FIGS. 2A to 2H are sectional views according to the respective processesfor explaining a method of manufacturing a semiconductor device inaccordance with a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, a preferred embodiment of a semiconductor device and itsmanufacturing method according to the present invention will bedescribed with reference to the accompanying drawings. In the followingdescription and drawings, the same reference numerals are used todesignate the same or similar components, and so repetition of thedescription on the same or similar components will be omitted.

In an embodiment of the present invention, a MOSFET exhibiting a SOI(Silicon on Insulator) characteristics is provided, in which anappropriate threshold voltage is obtained by, among other processes,depositing an electrode material to which an external voltage can beapplied to adjust the threshold voltage. This technique of the presentinvention enables the potential of the body region to be adjusteddirectly by the external voltage applied to the deposited electrodematerial.

In particular, after a trench etching process, a lateral etch process isperformed in a specific lateral direction to form a partial SOIstructure. On the laterally etched surface, an insulating film and aconductive electrode are formed. The voltage is externally applied tothe conductive electrode to adjust the potential of the body region, bywhich the threshold voltage of the MOSFET is adjusted.

These and other aspects of the present invention are explained in detailbelow.

As shown in FIG. 1, which is a sectional view of a semiconductor deviceformed according to a preferred embodiment of the present invention, thesemiconductor device includes a silicon substrate 1 having a deviceisolation film 13 for defining an active region, a gate 20 formed on thesilicon substrate 1, and junction regions (not shown) formed on asubstrate surface at both sides of the gate 20. The active region i.e.,the body region of the silicon substrate 1 is provided with a vacantspace 8 adjoining the device isolation film 13 within the active region.The surface of the vacant space 8 is formed with an conductive electrode11 a to which a voltage for adjusting the potential of a substrate bodyregion is externally applied while an insulating film 10 is interposedbetween the substrate body region and the conductive electrode 11 a.

The potential of the body region and thus the threshold voltage areeasily adjustable because the conductive electrode 11 a adjoins the bodyregion separated by the insulating film 10. As a result, thesemiconductor device of the present invention is capable of operating ata high speed, which is a typical characteristic of a semiconductordevice integrated in a SOI wafer, and the threshold voltage of theMOSFET can be adjusted very easily.

Now, a detailed description will be given for a method of manufacturinga semiconductor device in accordance with a preferred embodiment of thepresent invention with reference to FIGS. 2A to 2H.

Referring to FIG. 2A, a pad oxide film 2 and a pad nitride film 3 areformed on a silicon substrate 1 by, for example, a conventional STI(Shallow Trench Isolation) process. An organic anti-reflection film 4 isformed on the pad nitride film 3. Thereafter, a photoresist film pattern5 exposing the device isolation region is formed on the anti-reflectionfilm 4 by, for example, successive applications of coating, exposing,and development processes of photoresist film. Polymers such as COMA(Cycloolefin-Maleic Anhydride) or acrylate series may be used in formingthe photosensitive film.

In the exposed portion of the anti-reflection film 4 in the deviceisolation region, the pad oxide and nitride films thereunder and thesilicon substrate 1 are successively etched using the photoresist filmpattern 5 as an etch barrier to form a trench 6 now referring to FIG.2B. An insulating film 7 for a spacer is then deposited on the resultantsubstrate having the trench 6.

In forming the trench 6, a gas mixture of CF₄/CHF₃/O₂ may be used toetch the pad oxide film 2 and the nitride film 3. The flow rates of theCF₄, CHF₃, and O₂ gases are set to about 10 to 100 sccm, 10 to 300 sccm,and 10 to 70 sccm, respectively. A gas mixture of Cl₂/Hbr may be used toetch the silicon substrate 1. The flow rates of the Cl₂ and the HBrgases are set to about 10 to 100 sccm, respectively. Further, thetrenchs 6 in different devices may have different depths, since thetrench depth will depend largely on the degree of the device integrationIn general, the trench 6 is formed to a depth of about 1000 to 3000 Å,but it should be recognized that any trench depth, even outside therange given above, is possible in the present invention.

The insulating film 7 (for a spacer) is formed by an oxide film or anitride film but other insulating materials that are usually used in asemiconductor manufacturing process may be used instead. In particular,if a nitride film is used for the insulating film 7, it is possible todeposit an oxide film underneath the nitride film.

Referring to FIG. 2C, the insulating film 7 is etched to form aninsulating film spacer 7 a on the trench sidewalls including the exposedsurfaces of the substrate 1 and the etched pad oxide and nitride films2, 3.

The insulating film spacer 7 a is then used as an etch barrier in asubsequent process of etching the silicon substrate 1 in a specificlateral direction as will be described in detail below. That is, anopening process for exposing a substrate portion of the trench bottomsurface is further conducted in order to prepare subsequent wet or dryetch and oxidation processes in the manufacturing method of the presentinvention.

After forming the trench 6 with the lateral etching using the spacer 7 aas the etch barrier, a device isolation film 13 can be formed byperforming, for example, a sidewall oxidation process, a nitridedeposition process, a liner oxidation process, and a trench filling-upprocess.

Referring to FIG. 2D, the resultant substrate shown in FIG. 2C issubjected to a heat treatment at the temperature of 800 to 1000° C.under hydrogen atmosphere to remove foreign substances on the substratesurface. Thereafter, lateral etch in a specific lateral direction towardthe inner side of the insulating film spacer 7 a. The silicon substrate1 of the body region is exposed using the pad nitride film 3 and theinsulating film spacer 7 a as the etch barriers. In this way, a vacantspace 8 is provided in the active region of the silicon substrate 1.Referring back to FIG. 1, the insulating film 10 and the conductiveelectrode 11 a for adjusting a threshold voltage will be formed in thevacant space 8.

The lateral etching so as to provide the vacant space 8 may be performedby: a dry etch process using plasma; a wet etch process using chemicals;or a dry etch process using a gas mixture in which elements of GroupVII, such as F, Cl, Br or the like, and hydrogen are contained.

For example, the lateral etch for providing the vacant space 8 may beperformed using a gas mixture of HCl and H₂ at a temperature of 700 to1000° C., wherein the flow rates of the HCl and H₂ gases arerespectively set to 0.1 to 1 slm and 10 to 50 slm, so as to adjust anetch speed and an etch profile of the silicon substrate 1.

Referring to FIG. 2E, the resultant substrate is subjected to field stopimplantation to form an implant layer 9 within the substrate surface ofthe trench bottom surface. The formation of the ion implant layer 9 isintended to prevent leakage current between cells, which may be causedby applying a voltage to the electrode material for adjusting thethreshold voltage.

Referring to FIG. 2F, the insulating film spacer 7 a is removed and thenan insulating film 10 (for adjusting the threshold voltage together withthe conductive electrode 11 a to be formed later) is deposited on theresultant substrate. The insulating film 10 may be any insulating filmmaterial such as SiO₂, NO, ONO and HfO films, that are also usually usedas the gate oxide material.

Next, an electrode material film 11 for applying an external voltage isdeposited on the insulating film 10. Doped polysilicon is preferablyused as the electrode material film 11, but it is possible to use metalsuch as Al, Cu, etc., other than the polysilicon.

Referring to FIG. 2G, in order to remove the electrode material filmaround the active region of the silicon substrate 1, the resultantsubstrate is coated with a photoresist film 12. The photoresist film 12is then etched-back so that the photoresist film 12 is left only in thelateral-etched region of the the vacant space 8. Thereafter, an exposedportion of the electrode material film 11 is removed by a wet or dryetch technique using the remaining photoresist film 12 as the etchbarrier. In this manner, the conductive electrode 11 a, which is used toexternally apply a voltage to the body region, is formed.

Here, the electrode material film 11 may be etched using a gas mixtureof Cl₂/HBr, wherein the flow rates of the Cl₂ and HBr gases are set to10 to 100 sccm, respectively. The electrode material film 11 may also beetched using a solution mixture of acetic acid, nitric acid, and NH₃OH.

Although the photoresist film 12 is used as the etch barrier materialfor the removal of the electrode material film 11 around the substrateactive region in this embodiment, it is possible to use an oxide filmsuch as SOG, HTO, LTO, thermal oxide, and/or BPGS films instead of thephotoresist film.

Referring to FIG. 2H, the remaining photoresist film 12 is removed by astripping process, and the STI process is followed to form the deviceisolation film 13. In the process of filling up the trench with an oxidefilm or other suitable materials to form the device isolation film 13,voids may be formed in the laterally etched portion 8 of the trench dueto the structural peculiarity, but the voids, even if they are present,will not influence the adjustment of the threshold voltage.

Next, a gate 20 is formed on the silicon substrate 1. Here, it should beknown that the gate 20 has a multi-layer structure of a gate oxide film14, a gate conductive film 15 and a hard mask film 16 as shown in FIGS.1 and 2H. The gate 20 is also provided with a gate spacer 17.Thereafter, junction regions (not shown) are formed on the substratesurface at both side of the gate 20. In this manner, the MOSFET isconstructed.

Subsequently, a series of well-known DRAM manufacturing processes can beperformed to complete the semiconductor device.

As described above in respect of an embodiment of the present invention,a partial SOI structure is realized within the silicon substrate 1 byforming the insulating film 10 (and the conductive electrode 11 a) inthe laterally vacant space 8 as shown in FIGS. 1 and 2H. The potentialof the substrate body region can be easily adjusted by applying anexternal voltage to the separate conductive electrode 11 a. As a resultof this, the threshold voltage is easily adjustable to obtain a desiredvalue.

Accordingly, the electromagnetic fields in the junction and channelregions are reduced since the threshold voltage is adjustable by anexternal voltage without impurity implantation. This remarkably enhancesthe DRAM refresh characteristics and allows to realize high-speed andlow-voltage operations of the device.

Although preferred embodiments of the present invention have beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A semiconductor device having a silicon substrate having an activeregion between two device isolation films and a gate formed on thesurface of the active region, the semiconductor device having athreshold voltage that can be adjusted by the application of an externalvoltage to the substrate, the semiconductor device being comprised of:an unwardly extending silicon substrate having sides into which thereare formed laterally etched void portions in the active region below thesurface of the active region on the sides near the device isolationfilm; an insulating film formed on the laterally etched void portions ofthe silicon substrate; a conductive electrode formed on the insulatingfilm, the conductive electrode being sized, shaped and arranged toadjust a threshold voltage of the semiconductor device when a voltage isapplied to said conductive electrode; and a device isolation filmapplied on the conductive electrode, the device isolation film defininga cavity within the laterally etched void portions between the deviceisolation film and the conductive electrode.
 2. The semiconductor deviceof claim 1, further comprising a field stop implantation layer formed inthe substrate underneath the insulating film outside the active region.3. A semiconductor device having a silicon substrate having an activeregion between two device isolation films and a gate formed on thesurface of the active region, the semiconductor device having athreshold voltage that can be adjusted by the application of an externalvoltage to the substrate, the semiconductor device being comprised of:an upwardly extending silicon substrate having sides into which thereare formed laterally etched portions in the active region below thesurface of the active region on the sides near the device isolationfilm; an insulating film formed on the laterally etched portion of thesilicon substrate; a conductive electrode means for adjusting athreshold voltage of the semiconductor device when a voltage is appliedto said conductive electrode means, said conductive electrode meansbeing formed on the insulating film; and an isolation film applied onthe conductive electrode, the device isolation film defining a cavitywithin the laterally etched portions between the device isolation filmand the conductive electrode.